Generally, a character's dullness is equal to a viewer/reader's uncaringness. Riley, for example, is a BtVS character often slammed for being dull. But if a viewer steps into his head and tries to look at the story from his perspective - Riley's not dull. Tends to apply to most everyone.

So very true. I disliked Riley the second he showed up, so I needed a reason to hate him. 'Dull' is as good as any, plus it makes it sound as though hatred isn't involved at all. Which may just have been me, but if I did it, I'm sure at least one other person in fandom did, because no one is alone.